1. Field of the Invention
This invention is related to the processing of offset vertical seismic profile (VSP) data. More specifically, this, invention is related to the processing of gathered offset VSP data by sorting the offset VSP data into Zero Source Receiver (ZSR) common reflection point bins, correcting the sorted data for moveout and stacking the corrected data.
2. Description of the Prior Art
Vertical Seismic Profiling is a method for investigating underground formations by placing an acoustic wave detector in a well bored into the earth, placing a source of seismic energy such as an acoustic wave generator at a given distance from the well head, activating the source to generate acoustic waves into the underground formation under investigation and recording the resultant seismic signals produced by the acoustic detector. The acoustic wave detector is then moved to a series of different well depths, seismic energy is generated by the source for each placement of the acoustic detector at an additional borehole depth and the resultant seismic signals produced by the detector for each respective borehole depth are recorded. The detected signals recorded at the various depths are then grouped together in a single display for analysis. See, for example, U.S. Pat. No. 4,397,004 issued to Bronislaw Seeman for a description of Vertical Seismic Processing.
Borehole seismic acquisition methods are often designed to obtain data at multiple long offsets for a number of reasons. Some of the applications which require Vertical Seismic Profile data at multiple long offsets include: (a) providing offset seismic profiles recorded in a well that are directly correlative to well log data; (b) acquiring seismic data free of surface noise; (c) determining seismic transmission and reflection properties related to offset and depth; (d) acquiring data for tomography/inversion and imaging; (e) delineating reservoir properties when offset VSP profiles are conducted in a field for several wells with two-dimensional or three-dimensional applications; and (f) determining depth and inter-reflector velocities of reflectors below the borehole Total Depth (TD) from lookahead VSP data.
In many cases, Vertical Seismic Profiles must be conducted with the source placed at a considerable distance from the well. The reasons for the placement of the seismic source distant from the well are typically practical ones. For example, source-induced noise such as casing ring and tube waves can obscure reflection events when the source is close to the well. The configuration of the drilling support equipment on a well pad may also necessitate placement of the source at some distance from the well. Occasionally, culture or local land conditions dictate the placement of the source at a considerable distance from the well. Of these, the most frequent demand for long offset Vertical Seismic Profile data acquisition is the requirement for tube wave suppression. It has been found that the amplitude of tube waves diminishes with increasing source offset from a well. Also, the time delay of seismic reflection events resulting from a long offset source are usually less than the delay of tube wave inception. This tends to keep reflection events well separated from tube waves, resulting in much higher quality for data acquired during a VSP survey.
Offset VSP has not been as extensively developed as surface seismic processing of conventional VSP processing. Conventional VSP data processing assumes that the source offset from the well is essentially zero. Common Depth Point (CDP) stacking, a popular reflection seismology processing technique, utilizes the near-hyperbolicity of primary seismic reflection arrivals to align redundant reflections obtained from multiple shot/receiver experiments conducted on the earth surface. CDP stacking, however, has not produced similar results when used in conjunction with offset VSP data. The asymmetry of the shot and receiver locations used in offset VSP explorations precludes the possibility of CDP stacking to align offset VSP traces as exactly as CDP stacked traces of other types of seismic data.